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Sepam MCS025 Synchro-Check module - what is the usage of the "Measured voltages" setting (Usync1 / Usync2 or Vsync1 / Vsync2) in SFT2841?

This FAQ provides some details regarding the "Measured voltages" setting in the ANSI 25 protection function tab of SFT2841.

As per the Sepam user documentation, the MCS025 synchro-check module is designed to monitor two voltages on opposing sides of a breaking device in order to check if the 2 signals are sufficiently synchronized to allow the breaking device to be closed and the two parts of the power system to be coupled.

The user documentation included with versions of SFT2841 up to and including V14.1 does not describe in detail the usage of the "Measured voltages" setting that is provided in the settings tab of SFT2841 for the ANSI 25 protection function. The setting to be discussed in this FAQ document is shown in the screen capture below.

As described above, the MCS025 is designed to accept 2 separate voltage signals as inputs and to check if the two signals are sufficiently synchronized to allow for coupling of two parts of a network. These two voltages are typically taken from VT secondaries in the switchgear, and can be either phase-to-phase or phase-to-neutral voltages.

The purpose of the "Measured voltages" setting is to allow the user to define whether or not each signal applied to the MCS025 voltage inputs is a phase-to-phase or a phase-to-neutral signal.

It is important to note that, as shown in the screen capture above, the "Secondary voltage Uns Sync 1" (and "Sync 2") parameter is always defined in phase-to-phase volts. So, depending on which type of voltage has actually been wired or applied to the MCS025 voltage inputs, the settings will need to be adjusted accordingly. In order for the MCS025 to declare the applied voltages as being valid, the actual signal must be at least a certain percentage (defined by the "High Us" parameter) of the setting in order for "Close Enable" function to operate. Notice also that the "High Us" setting is at 85%, and "Secondary voltage Uns Sync1" setting is at 100V.

So, with respect to the "Measured voltages" setting and the sample values in the screen capture included above, let us consider 2 cases:

a) Case 1: Usync1/Usync2 setting is chosen. In this case, the MCS025 assumes that a phase-to-phase voltage is connected to the inputs, and it will check to make sure that the actual voltage applied is at least 85% of the "Secondary voltage Uns Sync1" setting (for our example, 100V). If the actual measured voltage drops well below 85% (defined by the "High Us" setting), then the Close Enable function will not operate, even if the 2 voltage signals are present.

b) Case 2: Vsync1 / Vsync2 is chosen. In this case, MCS025 assumes that a phase-to-neutral voltage is connected to the inputs, but it still uses the "Secondary voltage Usync1" setting as a phase-to-phase value. So, if the "Secondary voltage Usync1" is set to 100V, it means that the voltage applied to the MCS025 should be at least 85% of 58.13V (==> 100V / SQRT(3) ). If the voltage goes well above 85% of the setting, the Close Enable should still work. But, it cannot be significantly below the "High Us" threshold, after being converted to a phase-to-phase value and compared with the "Secondary voltage Uns Sync" setting.

Should you encounter a situation where the MCS025 module is not asserting the "Close Enable" output despite voltages having been applied to the voltage inputs for both "sides," then a value outside of the range defined by your "High Us" setting is the most likely reason for the problem.

To summarize, the "Measured voltages" setting should be programmed according the type of voltage signal that is wired to the MCS025 module. In addition to setting this parameter, you should ensure that the "Secondary voltage Uns Sync1" and "Secondary voltage Uns Sync2" settings have been programmed with the correct VT secondary phase-to-phase values.